Each of electric motors has a rotor and a stator. The stator has a stator core, in which motor coils are respectively wound on tooth sections covered with an insulator.
An electric motor for driving a vehicle, for example, is driven by low voltage and high current. Thus, thick cables whose diameter is, for example, 0.8 mm or more, are used as coil cables. In case of winding the thick cable on a tooth section, the cable is wound from a base part of the tooth section (an inner part of a slot) and lap-wound thereon. Then, the cable extended from an end of the coil is transferred to the next tooth section.
To form a level-wound motor coil, a winding-start point of a coil cable is set in a groove section, which is formed on a tooth section and extended from a base part of the tooth section to a front end part thereof, and wound on the tooth section. The cable is lap-wound from the front end part, and a winding-termination point of the cable is transferred from a middle part and wound on the next tooth section (see Japanese Laid-open Patent Publication No. P2007-221882A).
In another conventional example, a coil bobbin has a coil insertion groove, which is extended from an outermost part to an innermost part. A winding-start point of a coil cable is set in the coil insertion groove, a first layer of a coil cable is wound from the innermost part to the outermost part, a second layer of the coil cable is lap-wound from the outermost part to the innermost part, and a transfer cable is located on the outer side (see Japanese Laid-open Patent Publication No. P2007-89346A).
A typical stator of a conventional electric motor is shown in FIG. 6. In each of tooth sections 51, a winding-termination point of a motor coil 52 is located in a radially middle part of a slot 53, so a coil end is prone to be high. Since the winding-termination point is located in the middle part of the slot 53, the winding of the coil 52 is prone to be broken, by extending the cable for the next winding operation, so the cable cannot be level-wound and a lamination factor will be lowered.
In case of winding a thick cable on tooth sections 51 which have rectangular sections, the cable cannot be easily bent, so it is difficult to wind the thick cable. Further, a diameter of the winding must be increased, and gaps are easily formed in the winding section of the bobbin 54 and between layers of the cable. By forming the gaps, a lamination factor must be lowered.